Sheets ejected from an image forming apparatus such as a copying machine, a laser beam printer or the like sometimes need to be divided into sets so that each set may comprise a specified number of sheets. Various types of sheet storing apparatus for dividing a number of printed sheets into sets by shifting operation in the direction perpendicular to the ejection of sheets have been commercialized. Two ways of shifting operation for the sheets have been developed, one of them being that a tray is reciprocated and the other being that a pair of ejection rollers for transporting sheets onto the tray is reciprocated (Refer to U.S. Pat. No. 4,635,920).
However, in a conventional sheet storing apparatus, sheets stored on a tray are left alone without any means of aligning. Here is a problem that the sheets divided into sets by the shifting operation may be pushed by the next coming sheets and may be put out of alignment. Also, the shifting of the pair of ejection rollers may cause the complication of the constitution, and there is a problem that the pressure of the pair of ejection rollers is apt to be ill-balanced, so that the transport force on sheets may be scattered.